1. Field of the Invention
This invention pertains generally to a metalorganic chemical vapor deposition (MOCVD) and particularly to an apparatus and method for ultraviolet-visible in-situ monitoring and feedback control for a metalorganic precursor delivery used in MOCVD.
2. Description of the Related Art
Precise control of the relative partial pressures of metalorganics and hydrides is critical to the accurate and reproducible growth of epitaxial layers via metalorganic chemical vapor deposition (MOCVD). Strained-layered superlattices and narrow quantum well devices require accurate control over growth rate and layer doping to produce sharp, well defined boundaries between the component layers. Current MOCVD growth techniques employ calibration growth runs to determine reactant gas flow stabilization times and dwell times to flush the reactants and halt the growth process. Presently, metalorganic sources used in MOCVD of complex oxides are solids at room temperature and require elevated temperatures for sublimation. At these operating temperatures, volatility is unstable as a result of the chemical or physical changes in the original source material. For example, some compounds have been shown to thermally decompose upon heating or even oligomerize. As a result, reproducible film growth is difficult to achieve.
To provide data on the actual conditions within the growth chamber, several in-situ diagnostics techniques have been employed. One is the coherent antistokes Raman spectroscopy (CARS) which is used to monitor the partial pressure of AsH.sub.3 in a flowing cell and allows the determination of AsH.sub.3 concentration at a specific point in the reaction tube as a function of time. This technique, however, requires two high quality lasers and precise alignment, thereby reducing the cost effectiveness and utility for daily operation.
Another technique is infrared (IR) absorption spectroscopy of the metalorganics and hydrides utilizing an IR laser diode source with a multiple pass cell that can monitor partial pressures of the metalorganics and observe reaction products due to decomposition. However, the complexity of cell design, costliness of the measurement apparatus and perturbation of the growth environment preclude its widespread use.
Other methods have been utilized but they also have limitations. Some of these are flow studies using titanium oxide particles to characterize the flow conditions in the reaction chamber. However, the use of particles contaminates the growth system and requires an extensive cleaning prior to using reactant gases. Another, the ultraviolet detection method, is limited by the assuming of a constant molecular weight for the gaseous species. (Many of the precursors of interest form ologomers in the gas phase.) See, Hebner et al., In situ Measurement of the Metalorganic and Hydride Partial Pressures in a MOCVD Reactor Using Ultraviolet Absorption Spectroscopy, J. of Crystal Growth, Vol. 98, No. 3, pp. 293-301, 1989.